Hello,
due to my currently Project, I want to produce Micro Pillars made of PDMS. These are very small cylinders (D~10-20 micron and a height of ~200 micron) placed on a PDMS sheet. The sheet plays the role of a base and helps to get the pillars out of the negative mold. Afterwards, the sheet with the samples will be bonded inside a rectangular micro channel.
We will manufacture a SU-8 negative mold and are planning to cure the silicone in it. The next step would be the peeling off.
I have already tried to release specimens of simpler SU-8 molds and it turned out quite well, but with a relatively strong adhesion. For our pillar manufacture, we will need less adhesion force.
In order to make the peeling off easier (less adhesion), I thought about a treatment of the molds in order to make it more hydrophobe because of the hydrophilic behavior of the PDMS material. Users of researchgate on topic related questions suggested hydrophobe coatings and oils, but I am aware of the capillary effects of the very small holes in the mold. I will try it with some of these treatments and will share my experience later.
In order to make materials more hydrophilic (for a not period time, just a few hours would do a perfect job), there may be a possibility to O2 plasma treat the SU-8. It might work, but it is questionable if the treatment brings more errors into the structures then it helps with the adhesion. I found this article about the treatment of SU-8:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3370401/
Am I right with my theory and has somebody ever tried this kind of treatment with the production of PDMS specimens? There are a lot of people on researchgate having trouble with this problem.
Kind regards,
K.T.
Dear Karl,
In my opinion it is quite difficult to make your structures in SU8. It means you will have to make cavities in SU8 with an aspect ratio of 10-20. This requires a lot effort in finding the right exposure doses. Also the filling of PDMS might be tricky according to air entrapment. You might think of using a desiccator to get rid of this air after filling with PDMS. By the way, PDMS has a hydrophobic behaviour, not hydrophilic! Oxygen plasma of the SU8 will increase the adhesion of PDMS unless you coat it with a fluorocarbon layer after O2 plasma. I would suggest, to make the final structure (pillars) in SU8. Then replicate this in PDMS and use this as a master for a second PDMS. The master fabrication will be much easier!
Karl,
I agree with Daniel on this matter. I have significant experience in molding PDMS, including high aspect ratio structures similar to ones you are describing. I would recommend considering reactive ion etching (RIE) or deep reactive ion etching (DRIE) processes to make your mold if you are locked into your current approach.
However, if you should prefer to stick to SU-8 molding, I would recommend using a vapor vacuum deposition of perfluorinated silane (perfluoro trichloro silane is common AKA PFOCTS) to treat the mold. Typically, plasma treat the mold and place it with 2-3 drops of the silane in a vacuum desiccator (e.g. SpaceSaver) while pulling vacuum for 25-30 minutes.
Do not treat SU-8 with oxygen plasma to reduce adhesion. Plasma treatment of SU-8 will cause two problems: (1) high-strength oxygen plasma will etch SU-8 directly, and (2) lower strength plasma will oxidize the photoresist, allowing it to bond directly with uncured PDMS. This happens because surface oxidation exposes hydroxyl and carboxyl groups on the surface of the photoresist. If you pour uncured PDMS onto the surface, the siloxane backbone (Si-O) of the PDMS will form covalent bonds with the surface of the photoresist. Subsequent demolding will destroy the SU-8 mold.
Let me know if you have other questions.
We have used the passivation step from a standard DRIE Bosch process to improve release from high aspect ratio and small feature size PDMS micromolds. This surface modification resists hundreds of mould cycles in 80°C, where we found that silane based treatments only lasted 50 mould cycles.
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